Exploring oxygen diffusion mechanisms in Pr2NiO4+d under in situ conditions by neutron scattering: interplay between structure and lattice dynamics

• Applicant: Dr. Martin Meven
• Subject Area: Solid State and Surface Chemistry, Material Synthesis ; Experimental Condensed Matter Physics ; Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
• Term: from 2019 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 431446509

The project aims at studying the impact of lattice dynamics and oxygen ordering on the amplification of oxygen mobility at low temperature in the K2NiF4-type oxide Pr2NiO4+d. In particular, it will allow evaluating and extending a newly proposed phonon-assisted oxygen diffusion mechanism, enabling to rationalize oxygen mobility features in solid oxides, from room to moderate temperatures. If it can be confirmed that, at low temperature, oxygen mobility is essentially triggered by lattice instabilities combined with low energy phonon modes, this will have an important impact not only for a fundamental understanding of the diffusion mechanism, but also for the concept of designing and optimizing new oxygen ion conductors. It is obvious that this will strongly enhance their potential for a variety of technological applications such as oxygen membranes, sensors or catalysts, especially in the low temperature regime. Pr2NiO4+d presents a special case, as it shows a high oxygen diffusion rate at already moderate temperatures. It can reversibly take up a substantial amount of oxygen, yielding Pr2NiO4.25 as the maximum loading. The presence of intercalated oxygen atoms, which partially occupy the vacant interstitial lattice sites located inside the Pr2O2 rock salt layer, induces importantly local disorder resulting in large anisotropic displacements of the apical oxygen atoms. These displacements are supposed to dynamically trigger shallow oxygen diffusion pathwaysbetween the apical and interstitial sites via a phonon assisted diffusion mechanism. Our motivation here is to explore the specific role of the interstitial oxygen atoms on the oxygen diffusion mechanism in situ at moderate temperatures. Structure and lattice dynamics of non-stoichiometric Pr2NiO4+d will be investigated on single crystals, combining elastic and inelastic neutron scattering experiments. The use of a new mirror furnace, equipped with a quartz reaction chamber, allows adjusting the oxygen concentration in small increments, in a controlled gas atmosphere between RT and 950°C. Displacement amplitudes of the interstitial and apical oxygen atoms will be correlated with lattice dynamics as a function of the oxygen excess d and the temperature T. The final goal here is to identify oxygen order/disorder and associated formation of soft phonon modes to be a general prerequisite for low temperature oxygen diffusion mechanisms in solid oxides. This new concept will in fine allow extending and amending the classical Arrhenius Ansatz, which is currently used to describe ion mobility and associated activation energies at sufficiently high temperatures.

• DFG Programme: Research Grants
• International Connection: France
• Co-Investigator: Professor Dr. Georg Roth
• Cooperation Partners: Dr. Monica Ceretti; Privatdozent Dr. Antoine Villesuzanne